This invention relates generally to work allocation in an assembly line, and more specifically to operation assignment problems in a printed circuit (PC) board assembly process.
Along with the advent of powerful automated manufacturing systems has come a host of complex design and control problems. These problems include machine grouping, part type selection, operation assignment, fixture allocation, and tool loading. Some of these problems have already been considered in the prior art, such as the tool loading problem of Flexible Manufacturing Systems (FMS's). That problem is to allocate operations and required tools among groups of machines, while meeting technological and capacity constraints of the machines. The prior art suggested a number of different objectives, including balancing machine workloads and minimizing the number of movements of jobs between machines. One prior formulation of the FMS tool loading problem is an assignment problem with a linear objective--that of assigning job types to machines in order to minimize total variable processing cost per job, subject to machine capacity constraints.
Other prior formulations of the FMS tool loading problem consider the variable cost associated with each operation. In contrast, the assignment problem of the present invention includes not only variable processing cost per operation, but also a one time setup cost per job if any operations of a given job are carried out on a particular machine. This setup cost is incurred because of the particular technology we are considering, as described below. In addition, the FMS tool loading problem is a tactical problem that is concerned with objectives such as maximizing throughput or minimizing makespan, given a known set of jobs to be processed. Our assignment problem is a longer term planning problem that has the objective of minimizing expected production cost (or, equivalently, mibnimizing the average expected cost per unit produced) given estimates of expected future demand.
The problem solved by the present invention arose from conventional printed circuit (PC) board assembly operations wherein a unit assembles a number of different PC boards by inserting the appropriate components. The process is not fully automated because a wide mix of boards is produced, and the volume of production does not justy automation. Component insertion in this hand load cell can either be performed manually or by a semi-automated machine. Prior techniques did not adequately solve the assembly problems in this type of PC board manufacturing line.